Speckle imaging and interferometric techniques are utilized to provide an indication of the state of the surface of an object and are finding ever growing applications in science and industry. Speckle techniques rely upon the interference of a field of coherent light as it is reflected from an irregular surface of an object so as to produce an image characterized by a fine pattern of speckles Speckle images thus generated may be further processed to provide information relating to both the surface topology such as roughness, contours and the like as well as the vibrational modes of the object.
In many speckle imaging and speckle interferometric applications, there is a need to separate the coherent speckle image from a nonuniform non-coherent background. The non-coherent field may be generated by a non-coherent illuminating source, or it may be due to the decorrelation of a coherent incident field by an intervening medium, or by the physical characteristics, such as roughness or vibration, of the reflecting surface. For example, in speckle imaging, a coherently illuminated target may be imaged through a scattering medium where the scattering reduces significantly the contrast of the speckle image. Removing the non-coherent component in the received image field will recover the full contrast of the speckle image. Similarly, surface vibration of an object will decorrelate an incident coherent field and the vibrational modes of the object may be made distinct by removing the portion of the image field decorrelated by the surface motion.
Speckles are formed when a coherently illuminated diffuse or rough object is imaged with a finite imaging aperture. Due to the finite size of the point spread function or impulse response (i.e., the degree to which a true point source is focused by the system) of the imaging system, the optical field at a point in the image field is composed of a large number of scattered contributions with random amplitude and phase. When the contributions are fully coherent and with the same linear polarization, the optical field is the coherent sum of the complex amplitudes contributed by the scatterers within the point spread function. The resulting image of the extended diffuse object exhibits a random salt and pepper like pattern called speckle. The intensity characteristics of fully developed coherent speckles are described by a negative exponential intensity probability distribution and unity contrast where contrast is defined as the standard deviation of the intensity fluctuation divided by the mean.
With an non-coherent object field on the other hand, the contributions of the scatterers within the point spread do not interfere and they add their intensities. The image will not be speckled and for an object surface with uniform reflectance, the speckle contrast will be zero. For the intermediate case where the field is partially decorrelated or partially coherent, the speckle contrast will fall somewhere between 0 and 1 depending on the degree of coherence.
Previously employed techniques for removing the non-coherent component in the image field detect the difference in coherence by interfering the image field with a coherent reference beam. Typically, the resulting image field is detected, digitized and stored in a frame grabber on other memory device. Then the phase of the reference beam is shifted by .pi.. The phase shift produces no effect on the incoherent portion of the image field but it inverts the contrast of the coherent speckle image since constructive interference between the coherent image and the reference beam becomes destructive and visa versa. Subtracting the two frames removes the incoherent component which is unchanged between the two frames. What remains is a final image due only to the coherent portion of the detected image.
Such techniques are described in the following publications: K. Creath and G. A. Slettmoen "Vibration Observation Technique for Digital Speckle Pattern Interferometry" J.Opt. Soc. Am. A, 2, 1629-1636 (1985); K. Creath, "Digital Speckle Pattern Interferometry (DSPI) Using a 100.times.100 Imaging Array" Proc. Soc. Photo-Opt. Instrum. Eng., 501, 292 (1984); and O.J. Lokberg, J. T. Malmo and T. A. Slettmoen, "Interferometric Measurements of High Temperature Objects by Electronic Speckle Pattern Interferometry", Appl. Opt. 24, 3167-3172 (1985). Such heretofore available techniques for the reduction of non coherent image field function relatively well in laboratory situations. However, they are difficult to implement and utilize because of the need of employing several precisely aligned beams of coherent light as well as the need for phase shifting a reference beam between alternate image frames. Such a precisely disposed imaging system is very prone to vibrational interference from the instrument itself. Precise fringe monitoring is often necessary to ensure a high degree of mechanical stability. What is needed is a technique to remove the incoherent contribution in speckle imaging and interferometry which eliminates the need for a coherent reference beam thereby avoiding stability problems which have heretofore prevented widespread field use of such techniques.
The present invention provides method and apparatus for speckle imaging and speckle pattern interferometry which eliminates or substantially reduces the contributions of incoherent background illumination without the necessity for the use of a reference beam. It has been found that a pair of uncorrelated speckle images provided by the aperture sampling technique of the present invention may be appropriately processed so as to subtract background contributions. The present invention is particularly advantageous insofar as it provides a rugged system for speckle imaging, which can significantly reduce the incoherent background illumination and enhance the contrast of the speckle image or fringe pattern, which system is readily adapted for relatively high speed processing so as to allow for the production of real time imaging.